Tunable passive elements, such as capacitors and inductors, are used in a variety of radio frequency (RF) circuits to implement adjustable matching networks for antennas and power amplifiers, and to provide adjusting tuning for high frequency filters. Due to the high demand and production of portable devices, such tunable passive elements may be found in products such as cellular telephones, smart phones and portable computers. Providing tuning to RF circuits in such products allows these products to provide high performance RF transmission and reception in a variety of RF conditions. Programmable tuning is also helpful in RF devices that are configured to operate over different RF bands and/or configured to operate using different standards.
Tunable capacitors may be implemented in a number of ways. For example, a voltage-controlled capacitor may be used to provide a variable capacitance. Such a variable capacitance may be implemented using a reverse-biased diode junction having a capacitance that is inversely proportional to the applied reverse bias voltage. Another way in which a tunable capacitance may be implemented is by using an array of switchable capacitors whose various elements are either connected or disconnected via controllable switches. One challenge in the design of a switchable capacitor is dealing the effects of parasitic capacitance of transistors used to implement the switches. Such parasitic capacitances associated with the switching transistors may add additional capacitance to the switchable capacitance that may decrease the accuracy and the tuning range of the circuit and/or increase parasitic loading.